The prior art is aware that in the conditioning of natural and synthetic gases to remove the acid gases such as hydrogen sulfide. carbon dioxide, carbonyl sulfide and the like, that other acids, such as formic, sulfuric, sulfurous, thiocyanic, oxalic, chloric acids and the like. are generally also present in these gases and these acids form heat stable salts with the amine sorbents. These salts build up in the amine treating solution and must periodically be removed to maintain the overall efficiency of the amine with respect to regeneration for reuse in the absorbing process. The conventional manner for renewal of an amine sorbing solution which has become contaminated with sufficient amounts of the heat stable salts, and the like, to reduce its efficiency in pick-up of the conventionally referred to acid gases, is to transport the amine solution to a caustic treater wherein the salts are decomposed to their respective amine and acid components, the latter recovered from this process as the alkali salt of the acid. Such processes are time consuming, not readily adaptable to field unit operations on site of the absorber and are relatively expensive, particularly because of the need to transport the solution to be regenerated from the site to a caustic processing plant which to be economically viable must serve several absorber operations.
It has been known for considerable time that amine salts in general and those produced as a result of the gas conditioning of natural and synthetic gases could be regenerated bv electrochemical action. For example, Shapiro, U.S. Pat. No. 2,768,945, teaches one method for separation acidic gases from aqueous alkanolamine solutions used as absorbing solutions in the gas conditioning field. The Shapiro technique uses an electrochemical treatment of a portion of the thermally regenerated sorbing solution, a side stream, in a cell which separates the anode and cathode compartments from each other by use of a porous diaphragm. The anode is graphite and the cathode is steel. The anolyte is a weak acid and the catholyte is the amine solution. ln another patent, Kuo et al, U.S. Pat. No. 3,564,691 the electrolytio conversion of amine salts of the principle acid gases, such as hydrogen sulfide and oarbon dioxide, is described without mention of the effect of such electrochemical conversion of the other heat stable salts, viz., the amine formates, thiocynates, sulfates, sulfites, oxalates, chlorides and the like. This patent uses a multi-comPartment cell having at least one ion exchange resin-water compartment separating the electrode compartments each from intermediate compartments, which intermediate compartments include an acid compartment and product compartment, resPectively, and a central feed compartment, all defined by ion (cation or anion exchange) permeable membranes between compartments.
Neither of these processes is known to be used today, Shapiro being comparatively more expensive to operate than periodic purging of a portion of the sorbent and replenishment with virgin sorbent diluting the heat stable salt concentration to a level whereat the effect of the presence of the tied up (protonated) amine is minimized. Kuo et al is far too expensive to operate since a multiplicity of cells between electrodes increases the internal resistance of the cell increasing operating costs at least proportionally.
In co-pending application we disclosed an economical electrochemical cell and thus an economical conversion can be run in the field using the Shapiro scheme of side stream reclamation if the cell is designed as a simple two compartment cell employing specific materials of construction. Thus, an efficient cell can be produced when (a) a dimensionally stable transition metal oxide coated electrode is used as the anode, particularly iridium oxide oxide coated materials commonly used as anodes in conventional cells and (b) a single anion exchangg membrane, quaternerized functionalized polymers such as aminated polystyrene (e.g.; sold under the trademark lonics by lonics, Inc. or lonac by Sybron) separating the anode and cathode compartments. The cathode may be any suitable material having electroconductivity and stable under the use environment, e.g.; porous graphite, nickel and the like.
As disclosed therein experimentation carried out in our laboratories established that a graphite anode, as used by Shapiro, is a poor material since the current densities are low, and, while nickel and steel are very good with respect to current densities (they have high current densities) they lack the stability, they dissolve or corrode under the operating conditions. Other well known anode electrode materials, such as titanium and tantalum, have been shown to have low, on the order of graphite, operating current densities. Experimentation likewise has shown that an electrode coated with ruthenium oxide has the ability to operate at high current densities. but is not long-lived enough to be commercially viable under conditions normally found ln the field, since the ruthenium is worn away in about 30 days. Iridium oxide on titanium is shown in our co-pending Application. above identified, to operate at high current densities and to be sufficiently long lived to be commercially viable. Similarly, tantalum is expected to give equivalent results when coated with iridium oxide.